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Fault Displacement Hazard Evaluation for a Gas Transmission Pipeline: Holocene Rupture and Paleoslip Measurements Along Reverse Fault Splays Formed by a Restraining Bend in the Calaveras Fault

Session: Seismic Hazard Analysis for Critical Infrastructure

Type: Oral

Date: 4/19/2021

Presentation Time: 10:30 AM Pacific

Description: 

Pacific Gas and Electric Company maintains a Transmission Integrity Management Program (TIMP) to evaluate the threat of gas pipeline damage resulting from fault displacement. This TIMP investigation evaluated secondary fault strands adjacent to the Calaveras fault (CF) that cross a gas transmission pipeline in the southern San Francisco Bay Area. In the study area, the CF forms a 6-9° left-restraining bend north of San Felipe Lake. A series of NW-striking splay faults extending west of the main CF have been mapped based on W- to SW-facing scarps, closed depressions, and stream deflections. Detailed mapping and paleoseismic trenching were conducted along one of these splays 0.5 km SW of the main CF. At this location, the splay fault is expressed as a 12-m-high, W-facing scarp composed of Plio-Pleistocene Santa Clara Formation, and multiple inset alluvial fan surfaces are mapped at the base of the scarp. Two paleoseismic trenches across the scarp confirm it is composed of interbedded claystone, siltstone and conglomerate overlain by shallow colluvial deposits. Near the base of the scarp a NW-striking zone of low-angle (5-30ᵒ) thrust faults juxtapose Santa Clara Formation over fine-grain Holocene alluvium/colluvial deposits. Although timing is poorly constrained due to an absence of datable material, several stacked thrust duplexes suggest multiple surface-faulting events in the late-Pleistocene and Holocene. Two paleoslip measurements of 1.4 and 2.1 m are interpreted in one trench. We interpret the splay fault at this site to be a Holocene-active strand that ruptures during large-magnitude earthquakes on the main CF. If the interpreted single-event displacements on the reverse splay faults scale with displacement on the principal fault, the > 1 m displacement amounts suggest earthquakes of M > 6.5 on the CF.

Presenting Author: Robert W. Givler

Student Presenter: No

Authors